Cyanines pyrylium

Near infrared absorbers include cyanin compounds, pyrylium compounds, phthalocyanine compounds, and dithiol metal complexes. Antistatic agents include long chain alkyl alcohols and fatty acid esters with polyhydric alcohols. Stearyl alcohol and behenyl alcohol are the especially preferred compounds. Anti-fogging agents include sorbitan fatty acid esters and glycerin fatty acid esters. 

Strongly electron-releasing groups at the 2-, 4- and 6-positions of the pyrylium salts (39) exert a profound effect on the chemical shift of H(3) and H(5), which are moved upheld by 2-3 ppm. It is proposed that such compounds are better described as bridged pentamethine cyanines (40) (88MRC707). 

Two benzoannelated analogues of the pyrylium cation are known ben-zo[b]pyrylium (chromylium or 1-benzopyrylium) and benzo[c]pyrylium (isochromylium or 2-benzopyrylium) salts. So far, the 1-benzopyrylium system is considered to be the more interesting (79MI2 84MI1) because it is the basic heterocyclic system of important plant pigments (antho-cyanins), and because there are major differences between properties of its heterocyclic ring and those of monocyclic pyrylium systems as a result of benzoannelation. Thus, for instance, 1-benzopyrylium salts do not possess the ability to have recyclization reactions with heteroatom exchange. 

Fig. 3. Bimolecular electron transfer of photoexcited donors and acceptors. The parabolas (a) and (i>) represent Eq. 1 with l = 0.42 eV. Curve (a) is flattened at the top due to the diffusional limit [27]. The Rehm-Weller equations (Eq. 6) with identical A is presented by lines (e) and (/), where the former includes a diffusional limit of kd = 2 x 1010 M 1 s"The filled circles represent the Rehm-Weller data for neutral aromatic donors and acceptors in acetonitrile [25]. The squares denote similar data for inorganic (charged) acceptors and organic (neutral) donors also in acetonitrile [28], The data are fitted into Eq. 6 with X = 0.69 eV and kd = 9 x 109 M 1 s "1 (line d). The open squares represent the forward ET to the excited inorganic complexes the filled squares depict the bimolecular BET within the photogenerated ion pairs of the same systems [28], The triangles represent forward electron transfer between organic borates [29] and cyanines (filled triangles) or pyrylium cations (open triangles) within contact ion pairs in benzene. Even in this case, without diffusional interference, the data seem to fit better the Rehm-Weller equation (g) than the Marcus equation (c)...

Other studies that have been reported regard spectral and luminescent properties of some pyrylium and thiopyrylium salts (86MI1), emission properties of -y,y -chalcogenopyrylotrimethine cyanine dyes (90JA3845), and the effect of the polymethine chain length on the fluorescence spectra of symmetrical chalcogenopyrylocyanine dyes (92MI3). 

Symmetrical tetra-rert-butyl-substituted pyrylium and thiopyrylium mo-nomethine dyes have been shown by H NMR to be mainly present in solution in the conformation E,Z (29) and Z,Z (30), respectively (88KGS167) (Section II,B). Conformational analysis of monomethine and trimethine cyanine dyes containing pyrylium and thiopyrylium nuclei has been also carried out using nuclear Overhauser effect (89MI1). 

This behaviour may be due to the fact that the donor-substituted pyrylium ion 14 does not possess the structure of cyclic delocalized 671-systems, but rather that of a localized trimethine cyanine [2]. 

The side-chain reactivity of the pyrylium ion facilitates a targeted synthesis of cyanine dyes with pyrylium or 4//-pyran systems as terminal groups (see p 323). In this way, the dye 60 is produced by ViLSMElER formylation of the 4-methylpyrylium ion 58 to give the formylmethylene-4//-pyran 59 followed by aldol condensation with a second molecule of 58 [10] ... 

Applications as vehicles for organic syntheses are found, for instance, in the chemistry of pyrylium and pyridinium ions, e.g. syntheses for cyanines, azulenes, specifically substituted benzene derivatives and ring transformations to other six-membered heteroarenes with N, P and S atoms. 

SCHEME 10.3 Some examples of encapsulated species successfully prepared by in situ assembly of precursors inside the pores of zeolites, (a) Metal phthalo-cyanines (b) ruthenium tris-bipyridine (c) pyrylium cations (d) metal salen complexes and (e) heteropoly acid. The brackets generically represent the pore system of the solid host. 

The reactivity of 18 may be understood in terms of the electron distribution in the donor-substituted pyrylium ion, which apparently does not correspond to a cyclically delocalized 6jt-system (18a), but rather to that of a localized trimethine cyanine (18b/c). (6) Due to the charge distribution in the pyrylium system, CH3 groups in the positions 2, 4, and 6 display marked C-H-addity, since they are deprotonated by base. In the resulting enol ethers 20 or 22, the methylene groups can take part in electrophilic reactions of the aldol or Claisen type (side-chain reactivity, compare p. 358) ... 

---